This invention relates generally to an apparatus for polishing or planarizing a work piece, and more particularly to a wear ring for a polishing or planarizing apparatus that improves the uniformity of the polishing or planarizing operation.
The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. One commonly used technique for planarizing the surface of a work piece is the chemical mechanical planarization (CMP) process. In the CMP process a work piece, held by a work piece carrier head, is pressed against a moving polishing pad in the presence of a polishing slurry. The mechanical abrasion of the surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a planar surface.
The construction of the carrier head and the relative motion between the polishing pad and the carrier head have been extensively engineered in an attempt to achieve a uniform rate of removal of material across the surface of the work piece and hence to achieve the desired planar surface. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the work piece and accommodates variations in that surface. One or more pressure chambers may be provided behind the membrane so that different pressures can be applied to various locations on the back surface of the work piece to cause uniform polishing across the front surface of the work piece. The carrier head also generally includes a wear ring (sometimes referred to as a xe2x80x9cretaining ringxe2x80x9d or xe2x80x9cedge ringxe2x80x9d but hereinafter referred to without limitation as a xe2x80x9cwear ringxe2x80x9d) that surrounds the membrane and the work piece and that pre-stresses or pre-compresses the polishing pad to protect the leading edge of the work piece. The wear ring, which has an internal diameter (ID) only slightly larger than the diameter of the work piece, also serves to constrain the work piece, maintaining the work piece properly positioned under the carrier head. The polishing pad may move in a linear motion, a rotational motion, or an orbital motion, depending on the type of CMP apparatus. Additionally, the carrier head, and hence the work piece, may also be in rotational motion. The relative motion between the work piece and the polishing pad is designed to attempt to provide equal polishing to all areas of the polished surface. This relative motion, however, combined with the small but finite spacing between the outer edge of the work piece and the inner diameter of the wear ring also contributes to collisions between the work piece and the wear ring. These collisions can cause damage to the edge of the work piece and to the wear ring.
Despite all the efforts to achieve uniform polishing across a work piece surface, however, a uniform removal rate is not always obtained. Instead, in some instances a xe2x80x9cfast edgexe2x80x9d or high removal rate at the edge of the work piece is observed. This effect sacrifices potentially valuable work piece area around the edge of the work piece. In the case in which the work piece is a semiconductor wafer, that work piece area could be used to generate additional devices on the wafer.
Analytical modeling of the polishing operation has shown that controlled pre-compressing of the polishing pad with a pressurized wear ring can influence the material removal rate at the edge of the work piece. Analytical modeling has also shown that flexing of the wear ring near the edge of the work piece may cause inadequate or otherwise uncontrolled compression of the polishing pad and may lead to non-uniformity in polishing at the work piece edge. Erosion of the wear ring near the edge of the work piece may also cause a non uniform edge removal rate. Accordingly, a need exists for a wear ring that resists flexing and that can be used to achieve uniform polishing across the entire surface of a work piece.